Final answer:
Darwin's theory of evolution by natural selection involves heritable variation and environmental competition where traits offering survival and reproductive advantages become more common, explaining the diversity and unity of life.
Step-by-step explanation:
Darwin's theory of evolution by natural selection is founded on the concept of heritable variation and the process of natural selection. In his book On the Origin of Species, Darwin explained that evolutionary change is spurred by the differential survival and reproduction of individuals due to variations in their traits. These variations must be heritable so they can be passed on to the next generation.
Natural selection occurs because not all offspring in a population can survive; resources are limited, leading to competition. Offspring that inherit characteristics which provide an advantage in the competition for resources are more likely to survive and reproduce. Over time, these advantageous traits become more common within the population, leading to a process Darwin described as 'descent with modification'.
This evolution by natural selection ultimately results in populations that are better adapted to their specific environments, explaining both the unity and diversity of life. The idea that all life has evolved from a common ancestor through this process remains central to modern biology.